1. Field of the Invention
The present invention relates generally to the surface treatment of copper foil, and particularly to such treatments as are designed to render the surface of the copper foil more adherent, such as for printed circuit applications.
2. Description of the Prior Art
The use of copper foil in applications where adhesion to a non-metallic, or resinous substrate is required, is well known. For example, U.S. Pat. No. 4,088,547 to Albertson illustrates an application whereby copper foil of improved bond strength is used for the preparation of solar heat collectors. Most prevalently, however, adherent copper foil is used for the purpose of manufacturing printed circuits, wherein the foil is bonded to an appropriate resinous substrate. Numerous patents dealing with the surface treatment of copper foil to render it more adherent to plastic are known: for example, such pretreatments are disclosed in U.S. Pat. No. 3,220,897 to Conley et al.; U.S. Pat. No. 3,293,109 to Luce et al.; U.S. Pat. No. 3,585,010 to Luce et al.; U.S. Pat. No. 3,799,847 to Vladimirovna et al.; U.S. Pat. No. 3,857,681 to Yates et al.; U.S. Pat. No. 4,049,481 to Morisaki; U.S. Pat. No. Re. 29,820 to Konicek; U.S. Pat. No. Re. 30,180 to Wolski et al.; and WIPO Publication No. 8,202,991 to Torday
The foregoing patents are representative of disclosures that identify the difficulties encountered in the prior art, in the bonding of copper to synthetic substrates. In particular, the production of printed electronic circuits involve the bonding of the copper foil to the polymeric substrate, followed by the selective removal of portions of the copper foil to define the actual electronic circuitry. The foil is generally selectively removed by acid etching, whereupon a desired metal pattern is achieved. One of the problems involved in the formation of printed circuits, however, has been a tendency on the part of the copper foil to exhibit inferior bonding capability with the resin substrate, with the result that the final circuit board is commercially unacceptable. Frequent difficulties comprise delamination or even fracture of the copper layer. The prior art relates that numerous efforts to improve bonding have included the application of novel adhesives and efforts to pretreat the bonding surface of the copper foil to render it more adherent.
As pointed out in U.S. Pat. No. Re. 30,180, efforts to improve bond strength of copper have involved the deposition on the surface of the copper foil of a layer of "nodular" copper, to increase surface area and roughness. This deposition has usually required the subsequent application of a layer of pure copper or the like, to "lock" the particulate copper/copper oxide on to the surface of the foil, and to prevent otherwise disadvantageous powder transfer. While these treatments have been tried in their many variations, as set forth in representative form in the above listed patents, none have succeeded in simultaneously optimizing bond strength and economy of preparation. Thus, early efforts to increase bond strength comprised the deposition of additional quantities of nodular copper, however this approach resulted in powder and oxide transfer problems. Efforts to avoid such transfer problems centered around decreasing the thickness of the nodular copper electrodeposit, which consequently caused an undesirable loss in bond strength.
Of the patents listed above, Applicant notes that the most prevalent techniques for the treatment of copper foil are disclosed in the '109 Patent to Luce et al. and the U.S. Pat. No. Re. 30,180 to Wolski et al. In each of these patents, an attempt to improve bond strength of copper is made by the application of a first "roughening treatment", whereby particulate or "dendritic" copper is applied to the surface of the foil, and a second "locking" or "gilding" treatment whereby a smooth layer of copper is electrodeposited over the first roughened surface to secure it to the foil substrate. In the Luce et al. technique, each treatment utilizes a bath of different composition, and the bath wherein the first or roughening treatment is applied, must critically contain a proteinaceous material such as animal hide glue, to control the nature of the deposition of the first nodular layer. In the Reissue Patent to Wolski et al., the first layer is prepared from a bath containing copper and arsenic, the arsenic added to improve bond strength of the deposited layer. The Wolski et al. technique includes a third surface treatment to apply a further electrodeposited microcrystalline copper and arsenic-containing layer.
The techniques described above have been investigated for the purpose of developing a commercially attractive copper foil product, and were found to be undesirably complex and expensive to practice. The requirement of separate treating baths and the constant replenishment of the relatively expensive ingredients presented drawbacks. Also, tank contamination frequently occurred when the foil was transferred between the separate treatment baths, and the added step of rinsing the foil was required to alleviate this problem. The use of the rinse(s) also placed an increased burden on pollution control.
In the earlier work by the present inventor, disclosed in copending application Ser. No. 602,842, the disclosure of which is incorporated herein by reference, the prior art techniques were distinguished by the development of a method wherein copper foil was treated in a single treatment bath consisting essentially of an aqueous solution of copper sulfate and sulfuric acid, wherein the copper foil was subjected to a multitude of treatment cycles wherein electric current is applied while the foil remains immersed in the solution, the first application of current conducted at a magnitude and duration sufficient to deposit a first nodular layer on the surface of the copper foil, with the second application of electric current of a magnitude less than that of the first, to form a second layer of copper thereover. This technique contemplated a multitude of such treatment cycles to develop in the final treated copper foil a coating offering substantially improved bonding characteristics and freedom from undesirable treatment transfer. While this technique is capable of achieving bond strengths on the order of 17 to 18 pounds per inch width for 2-ounce copper foil, it was theorized that even this technique might be further improved to achieve greater bond strengths with reduced input in total treatment time and corresponding expense of equipment and materials. To this end, the previous work and method just described was evaluated to determine those modifications both in theory and in practice, that might make possible the achievement of these improved attributes and results.
The present application is therefore an outgrowth of the additional work by the inventor that has culminated in a further and redefined treatment method.